The sitting of wind farms has a direct impact on its operating efficiency and profitability and it is therefore decided taking into account among other facts the results of extensive wind speed measurements recorded in apparatus generally known as data loggers.
Known monitoring systems use wind speed measurements at sampling time intervals of some seconds or less at a proposed site and data loggers store the wind speed mean and the wind speed standard deviation for periods of a predetermined duration, typically 10 minutes. These statistical data are then used for evaluating the suitability of the site for installing one or more wind turbines. The basic wind speed time-series are not stored in data loggers.
For evaluating the suitability of a site for a wind farm, the knowledge of the turbulence intensity at the site is very important. Wind turbines are subjected to a big amount of loads during its operating life (20 years) and most of them are aerodynamic loads due to wind intensity whose variation produce fatigue on the wind turbine components.
The variation of the wind speed is distributed according an spectrum of cycles at different frequencies depending on the different size and duration of the gusts.
The turbulence intensity in a given period is obtained dividing the wind speed standard deviation by the wind speed mean in the period:T.I.=σv/Vmean 
The standard deviation of the wind speed time-series σv represents the integral effect of the varying frequency fluctuations:
      σ    v    =            ∫      0      ∞        ⁢                            S          v                ⁡                  (          f          )                    ⁢                          ⁢              ⅆ        f            
Unfortunately, the measured standard deviation of the wind speed time-series also includes the very low frequency variation corresponding to a more or less continuous rise or decrease of the wind speed (wind speed trend) in the 10 minutes period that can not be suitably considered as turbulence and should be removed.
So, the obtaining of a suitable turbulence intensity should consist on calculating the fitted straight line that correspond to the trend line and subtract it to the original time-series.
The trend is very site dependent and result in higher values for places with an oceanic climate where frontal passages are very frequent.
Although it is known that the fatigue on the wind turbine components is mainly dependant on the de-trended turbulence intensity because the trend part of the turbulence intensity affects to low frequency fluctuations that are not prejudicial for the wind turbine, the known systems for monitoring the wind characteristics in a proposed site for a wind farm are not able to eliminate the trended part of the turbulence intensity.
The present invention is intended to solve that drawback.